The old master and apprentice craft of matte painting is dissolving. Today most of us can declare ourselves a matte painter, and get a job. many of the people I admire in the business did this on glass, and learned from Whitlock and Ellenshaw. Those two artists are dead, and others of their generation are getting on in years.

Would that their apprentices speak up.

How do you create a held take shot? What about YCM painting or color separations in rear projection? How did they move on paintings before digital? Matte Scan? There were systems before that which did so.

Composition? Color balance? Best practices.

If such a compendium of knowledge existed, what is that worth to the matte-pasters of today?

What would knowledge of painting from Matisse been worth if he wrote it down? This is essentially where we are now.

I'm surprised Dave Mattingly didn't respond to your post. Although, then again, he probably didn't see it. David worked at Disney, during the Dick Tracy period. I think both Harrison and Peter Ellenshaw were involved, as well as others like Michele Moen, and Paul Lasaine. They all have personal websites/blogs, so there's a starting point for you if you're interested in learning. Also, StopMotionAnimation.com has a dedicated group of people to matte painting, pre-digital era. Mark Sullivan, and Rocco Gioffre sometimes respond to posts over there. David Stipes as well. Jim Danforth used to be a member there but I don't think he is anymore. I'm pretty sure Mattepainting.org is an all digital arena, and while its members can appreciate 'old school' matte work, they probably can't comment on the technique at great length like you're looking for, except for a couple guys like David who is a member here. Craig Barron used to visit, and he's a good resource for pre-digital stuff too.

If you have a way to reach Director James Cameron, he could elaborate for you as well. Did you know he did some old school matte work himself?